1. Field of the Invention
The present invention relates to electrochemical cells and methods for their manufacture. More particularly, the invention relates to microfabricated electrochemical device separators, in particular for double layer capacitor cells, which provide improved cell performance including decreased impedance.
2. Description of the Related Art
Unlike a battery, a capacitor cannot produce electrons by chemical reaction. However, capacitors, have a distinct advantage over batteries in certain applications in that that they can be recharged incrementally and quickly. Thus, capacitors are advantageously used in hybrid systems in conjunction with batteries to support brief high current demands through pulse discharges. Also, they may be used for power storage and as stand-alone power back-ups, for example, to provide a stable power source for computer memory in the event of a primary power source failure.
There are two principal types of capacitors: Parallel plate, and double layer. A parallel plate capacitor is composed of two electrically conductive (typically metal) plates (electrodes) separated by a dielectric. A difference in charge on the two electrodes is maintained by the dielectric between them giving rise to capacitance.
Double layer capacitors include at least one inert, high surface-area electrode and an electrolyte. Their capacitance arises from a separation of charged species at the electrolyte-electrode interface (i.e., the xe2x80x9cdouble layerxe2x80x9d). Typically the charge separation distance is on the order of atomic dimensions. The capacitors are often referred to as xe2x80x9csupercapacitorsxe2x80x9d because of their potentially enormous capacitance (possibly as high as farads or tens of farads per gram). This high capacitance results from a combination of the very small charge separation distance inherent in double layers coupled with a particularly high surface area of the electrodes (often on the order of thousands of meters squared per gram).
A typical double layer capacitor cell contains two electrodes separated by a dielectric (typically an electrode separator) to maintain electrical isolation between the electrodes. These double layer capacitors are charged by applying a voltage across the two electrodes and thereby increasing the total charge stored in the double layer of each electrode. The voltage difference across the two electrodes can continue to increase during charge so long as the leakage current between the electrodes remains low. Ultimately, however, the potential difference between the two electrodes will reach a level where the electrolyte solvent is electrolyzed (introducing a large source of leakage current). Generally, electrolysis of the electrolyte is to be avoided in double layer capacitors because it can cause, among other detrimental effects, loss of electrolyte, increased pressure in the cell, and formation of explosive mixtures (oxygen and hydrogen in the case of aqueous electrolytes).
Conventional double layer capacitor devices are typically packaged in a metal container. FIGS. 1A and 1B illustrate simplified examples of such packages:
FIG. 1A shows a conventional double layer capacitor device package of a metal can with protruding leads. The capacitor 100 is composed of a pair of electrodes 102, 104 separated by a separator material 106 and wound into a roll 109. The separator 106 is typically a porous electrically insulting material, such as polyethylene. An electrolyte is also present between the two electrodes, typically permeating the separator. The purpose of the electrolyte in a double layer capacitor is to provide ion transport between the electrodes. Electrolytes for this purpose may be aqueous or organic. Suitable aqueous electrolytes include dilute acids and alkaline solutions such as 30% sulfuric acid or 40% potassium hydroxide. Suitable organic electrolytes include quarternary ammonium salts dissolved in a solvent such as propylene or ethylene carbonate. The capacitor components are contained in a metal, typically aluminum or steel, can 108. Leads 110 connected to the two electrodes 102, 104 protrude from the can 108 for external electrical connection.
FIG. 1B shows a conventional double layer capacitor device package of a coin cell. The capacitor 120 is again composed of a pair of electrodes 122, 124, in this case disks, separated by a separator material 126. The separator 126 is typically a porous electrically insulting material, such as polyethylene. An electrolyte is also present between the two electrodes, typically permeating the separator. The capacitor components are contained in a metal, typically aluminum or steel, package 128 composed of a case 130 and a cap 132. In this package, rather than protruding leads, where the metal package itself acts as a lead for external electrical connection.
Conventional separator materials for these electrochemical devices are paper, porous polymer (e.g., polyethylene) materials, and glass fiber mats. These materials have typical minimal thicknesses of about 30-80 microns, 8-9 microns, and 200 microns, respectively. The porosity of paper and polymer separators is typically about 30-50% and that of glass fiber may be as much as 80%. These conventional separator materials also each have an element of xe2x80x9ctortuosity,xe2x80x9d that is, the degree to which the pores (or other paths through the materials) depart from a straight line. The greater the tortuosity of a porous material, the more the paths, on average, depart from a straight line, and the more resistance there is to passage of electrolyte, salts and ions through the separator material.
In a multi-electrode double layer capacitor cell, the migration of ions in the electrolyte from one electrode interface to the other occurs during charging and discharging of the device. Efficient ionic transport means lower impedance resulting in faster charge and higher power delivery. The less efficient the ionic transport through the separator, the higher the impedance of the device. Also, cell impedance is affected by the distance between the electrodes. The greater the distance between the electrodes, the less efficient the ionic transport and the greater the impedance of the cell. Further, decreasing the distance between the electrodes in a multi-electrode double layer capacitor cell would allow for the incorporation of more active material (i.e., carbon) in a given cell volume so that the volumetric capacitance (e.g., farads (F)/cc) might be increased beyond the 1 to 5 F/cc of state of the art commercial products.
Accordingly, in order to produce lower impedance double layer capacitor cells and to increase volumetric capacitance, it would be desirable to reduce the distance between electrodes in double layer capacitors and to increase the proportion of the area between the electrodes available for ionic transport while facilitating the manufacturing of the cells.
The present invention addresses this need by providing electrochemical (e.g., double layer capacitor) cell designs and methods of their manufacture, which reduce cell impedance and increase volumetric capacitance while maintaining inter-electrode dielectric integrity and cell performance and facilitating manufacturing. The designs adapt mircofabrication techniques from the field of semiconductor fabrication in order to form and pattern thin dielectric films on electrodes. Existing microfabrication techniques allow for the formation of dielectric (e.g., polyimide) films having a thickness of about 1 to 2 microns. Dielectric films formed on electrodes may be patterned according to well known procedures in the semiconductor fabrication field to provide area for unimpeded ion exchange between the electrodes. The patterning may produce contiguous or noncontiguous dielectric layers between the electrodes having porosity of about 30 to 80%, preferably about 60 to 80% while dielectric integrity is maintained. The result is a lower impedance, higher performance, easily fabricated double layer capacitor cell.
In one aspect, the invention pertains to an electrochemical cell. The cell includes a cell container, a first electrode provided within the cell container, a microfabricated porous dielectric disposed on the first electrode, a second electrode provided within the cell container disposed adjacent to the microfabricated porous dielectric such that the microfabricated porous dielectric provides electrical isolation of the electrodes, and an electrolyte provided within the cell container. Electrochemical structures incorporating microfabricated porous dielectrics that may be used as cell components are also provided.
In another aspect, the invention pertains to a method of making an electrochemical cell. The method involves microfabricating a dielectric layer on a first electrode, positioning a second electrode adjacent to the microfabricated dielectric layer, placing the electrodes in a cell container, providing an electrolyte in the cell container, laminating the electrode and dielectric layers, and sealing the cell.
These and other features and advantages of the present invention are described below with reference to the drawings.